The chemical, immunochemical, and physical properties of cell-surface glycoproteins in immunosensitive cell lines and the topographies of these lines, as visualized by scanning, transmission, freeze-etch, and freeze-fracture electron microscopic techniques, will be compared with those of immunoresistant cell lines of the same murine tumors. Studies will involve spontaneously-developed mammary carcinoma ascites cells which become immunoresistant spontaneously and those which were selected for immunoresistant characteristics; Moloney virus-induced lymphoma cells which have been immunoselected for resistance to antiserum to the Moloney virus-determined cell-surface antigen and those which have been selected for resistance to anti-histocompatibility (H-2) antiserum. Isotopically-labelled cell-surface components believed to be implicated in escape mechanisms, including tumor rejection antigens and macromolecules involved in masking or modifying antigenic activities, will be removed from the cells, purified, and studied for immunological properties and for detailed chemical structures, as determined by chemical, enzymatic, and immunochemical techniques.